The invention pertains to the art of heat pump systems in which staged compressors are used in a series arrangement and in which under normal operating conditions both of the compressors run when the system is operating. Since both compressors are started with across-the-line starters and it is therefore undesirable to start them both at once, the control of the invention is directed to an arrangement in which the sequential starting of the two compressors is obtained within a few seconds but with neither compressor continuing to operate if the other, for some reason, fails to start and run.
It is of course known to provide heat pump systems and refrigeration systems with multiple compressors and to arrange them so that, to the extent more than one will operate, they are brought onto the line in sequence. However, so far as we know these arrangements are typically of the type in which a second or third compressor is brought onto the line only when needed so that there is no requirement that the second or third compressor be brought onto the line for the first compressor to continue to operate. In some of these patents, such as U.S. Pat. Nos. 2,434,221 and 2,453,095 the second compressors are brought on line in accordance with pressure conditions in the refrigeration system and without the provision for time delays or a requirement that both compressors operate if either is to operate.
U.S. Pat. No. 3,599,006 discloses a control arrangement for a cascade refrigeration system in which three compressors are brought onto the line in sequence, separated by about 10 seconds each, but without an arrangement for making their operation interdependent upon each other as is provided in our invention.
U.S. Pat. No. 3,668,883 also discloses multiple compressors but again the control arrangement is such that the main compressor is capable of operating alone and the booster compressor is only brought on line as needed.
The aim of our invention is to provide a heat pump system with series arranged compressors and a control arrangement which provides for starting of the compressors in close sequence with the shutdown of the system if either compressor should fail to start and run.